Having produced T-cell hybrids which secrete a product which binds to the arsonatehapten, and which can be propagated both in culture as well as in the ascites form, we wish to characterize this product both chemically and functionally. The primary focus of this application is on the immunochemical characterization of these T-cell products, although functional analyses will be performed in collaboration with Drs. Kettman and Forman of our department. Preliminary studies indicate that a single 62,000 dalton polypeptide not only bears determinants encoded by the I region of the murine major histocompatibility complex, but also binds antigen (arsonate) as well as bears determinants related to the arsonate cross-reacting idiotype. Genetic and chemical analysis of these molecules should provide information concerning T-cell products which are undoubtedly important in immunoregulation.